Electrothermal timing apparatus



July 22, 1969 F. P. BUITING 3,457,429

ELECTROTHERMAL TIMING APPARATUS Filed Jan. 9, 1967 2 Sheets-$beet 1F/Zam 'o 7 ZSM-zi l ow v, W 7 mJLzawctt,

July 22, 1969 F. P. BUITING I ELECTROTHERMAL TIMING APPARATUS 2Sheets-Sheet 2 Filed Jan. 9, 1967 swz United States Patent 3,457,429ELECTROTHERMAL TIMING APPARATUS Francis P. Buiting, Plainville, Mass.

(34 Forest St., Attleboro, Mass. 02703) Filed Jan. 9, 1967, Ser. No.608,230 Int. Cl. HOZh 3/00; H02!) 1 24; H01h 7/06 U.S. Cl. 30741 12Claims ABSTRACT OF THE DISCLOSURE Electrothermal timing apparatus isdisclosed in which a plurality of pairs of thermistors pass throughelectrothermal cycles in sequence. In each pair the thermistors are ofopposite temperature coefiicient types and are interconnected in acircuit such that heating of one of the thermistors will initiate anelectrothermal cycle in which one and then the other of the thermistorspass through self-heating phases in sequence. Successive pairs arethermally coupled so that the occurrence of an electrothermal cycle inone pair will initiate a similar cycle in a subsequent pair.

.This invention relates to electrothermal timing apparatus, and moreparticularly to such apparatus which is suited for determiningrelatively long periods. 7

It has heretofore been known to employ thermistors for timing purposesand to use pluralities of electrically interconnected thermistors forobtaining oscillatory or sequencing functions. However, in these knownconstructions, the heat dissipated from the thermistors is typicallytreated only as a power loss. While the heat losses are accounted for inthat they interrelate with and determine the electrical characteristicsof the thermistors, the losses are themselves not employed for anyuseful purpose. In many of the prior art sequencing devices in whichpluralities of thermistors were interconnected, each stage typically hadto-be operated at a lower power level than the one preceding it, sinceeach stage drew its electrical power through the preceding stage.

Among the several objects of the present invention may be noted theprovision of novel electrothermal timing apparatus employingself-heating thermistors for obtaining a time delay; the provision ofsuch apparatus in which the heat resistively dissipated in at least someof the thermis-' tors is usefully employed to effect signal couplingbetween successive timing stages; the provision of such timing apparatusin which successive stages may be operated at similar power levels; theprovision of such apparatus in which successive stages may beelectrically isolated; the provision of such timing apparatus which issuited for determining relatively long periods; the provision of suchapparatus in which such successive stages may operate at widelydissimilar time intervals; the provision of such timing apparatus whichcan time programming functions; the provision of such timing apparatuswhich can be constructed simply and inexpensively; and the provision ofsuch timing apparatus which is highly reliable. Other objects will be inpart apparent and in part pointed out hereinafter.

Briefly, electrothermal timing apparatus of this invention involves aplurality of pairs of thermistors. One of the thermistors in each of thepairs is of a first temperature coefiicient type and the other is of theopposite temperature coefficient type. The thermistors in each pair areinterconnected with each other to provide electrical couplingtherebetween and the different pairs of thermistors are interconnectedwith an electric power source whereby heating of the thermistor of thefirst coefficient type in one of the pairs will initiate anelectrothermal cycle in which the one and then the other of thethermistors in 3,457,429 Patented July 22, 1969 lCC that one pair passthrough self-heating phases in sequence. The thermistor of the oppositecoeflicient in at least one of the pairs is thermally coupled to thethermistor of the first coefficient type in another one of the pairswhereby the occurrence of an electrothermal cycle in the one pair ofthermistors initiates an electrothermal cycle in the other pair ofthermistors.

The invention accordingly comprises the constructions hereinafterdescribed, the scope of the invention being indicated in the followingclaims.

In the accompanying drawings in which several of various possibleembodiments of the invention are illustrated:

FIG. 1 is a schematic circuit diagram of electrothermal timing apparatusof this invention employing a plurality of pairs of thermistors, eachpair of which includes a PTC and an NTC thermistor;

FIG. 2 is a graph representing the changes in resistance of the PTCthermistors with respect to changes in temperature;

FIG. 3 is a graph similarly representing the changes in resistance ofthe NTC thermistors with respect to changes in temperature;

FIG. 4 is a graph representing the current/voltage behavior of the PTCthermistors;

FIG. 5 is a graph representing the voltage/current behavior of the NTCthermistors; I

FIG. 6 is a schematic circuit diagram of a modification of thethermistor timing apparatus of FIG. 1;

FIG. 7 is a side view of a thermistor pair construction suited for usein the apparatus of FIG. 6;

FIG. 8 is a top view of the thermistor pair construction of FIG. 7; and

FIG. 9 is a schematic circuit diagram of electrothermal timing apparatusemploying pairs of thermistors in which the pairs are connected inseries.

Corresponding reference characters indicate correspending partsthroughout the several views of the drawmgs.

Referring now to FIG. 1, the timing apparatus illustrated there includesa plurality of pairs (1118) of thermistors. Each pair comprises abistable switching stage and includes one thermistor having a positivetemperature i PTC thermistor and the NTC thermistor constituting eachpair are connected in series with each other across a pair of supplyleads L1 and L2 through which electrical power at substantially constantvoltage is supplied to the apparatus. Since the thermal behavior of thevarious thermistors is governed essentially by the power dissipatedtherein, direct current of either polarity may be applied to the leadsL1 and L2 or alternating current of equivalent RMS voltage may be used.

A PTC triggering thermistor PT1 is connected across leads L1 and L2through a circuit which includes a current limiting resistor R1 and anormally open push-button switch PBl. When switch PB1 is closedthermistor PT1 heats to its transition temperature. 'Ihermistor PT1functions as an external heater and is thermally coupled to NTCthermistor N11 as indicated by the dotted line linking the twothermistors. Thus, by closing switch PBI, thermistor N11 can be heatedby thermistor PT1.

The FTC thermistors P11-P17 of the pairs 11-17 are thermally coupled tothe NTC thermistors N12N18 respectively in the pairs 12-18 as indicatedby the dotted lines enclosing the respectively coupled thermistors. Thiscoupling may, for example, be provided by electrically insulatedmetallic strips. Thus, if PTC thermistor P11 self- 3 beats, heat will beapplied to NT C thermistor N12 by conduction through the thermalcoupling.

Preferably, the PTC thermistors P11-18 are of the type having a sharplydefined transition temperature above which the thermistor materialsresistance increases sharply. An example of a material which possessessuch a resistance characteristic is doped barium titanate (BaTiO Thebehavior of the resistance of this material is represented in FIG. 2with respect to changes in temperature, the transition temperature beingindicated on the temperature scale at TR.

Thermistors constructed of this material have an equilibriurncurrent/voltage characteristic, as illustrated in FIG. 4, which has apronounced negative resistance region as indicated at NRP, that is, theequilibrium current passing through the thermistor decreases withincreasing voltage levels above a predetermined threshold. As 1sunderstood by those skilled in the art, devices hav ng negativeresistance characteristics are capable of being connected in circuitsfor producing bistable switching. It should be understood that thecurves of FIGS. 4 and 5 represent the behavior of the respectivethermistor element when it has sufficient time to come to equilibriumunder a given set of ambient conditions and that the shape and size ofthe curve may be altered by varying the ambient conditions as, forexample, by applying heat to the thermistor element.

The resistance characteristicsof the NTC thermistors N11-N18 arerepresented in the graph of FIG. 3. As may be seen, the resistancedecreases in a gentle curve with increasing temperature. Thevoltage/current characteristic of the NTC thermistors N11-N18 isrepresented in the graph of FIG. 5. As may be seen, the NTC thermistorsN11-N18 also have a negative resistance region as indicated at NRN inwhich the voltage across the thermistors falls with increasing currentabove a predetermined threshold. This negative resistance region is,however, typically not as large nor as steep as that possessed by thePTC thermistors. It should be noted that the voltage and current scaleshave been reversed in FIG. 5 as compared with FIG. 4. It may thus beseen that the NTC thermistors are, in certain respects, current analogsof the voltage characteristics of the PTC therm1stors.

While NTC thermistors having gently sloped resistance characteristicsand PTC thermistors having steeply sloped characteristics areillustrated herein by way of example, it should be understood that steepslope NTC and shallow slope PTC thermistors may also be employed undervarious conditions.

' When thermistors having opposite temperature coeflicients ofresistivity are connected in series across a source of substantiallyconstant voltage, electrical interaction between the two thermistors isobtained. Such series-connected thermistors are essentially voltagecoupled m that, if the voltage across one of them changes, the voltageacross the other must change in complementary fashion. In the apparatusillustrated in FIG. 1, the voltage applied to the leads L1, L2 isselected in relation to the characteristics of the particularthermistors to obtain a bistable or two-state switching mode ofoperation in which one and then the other of the thermistors in a pairpass through self-heating phases when that pair is triggered.

While a theory of operation for the thermistor pairs has not beencompletely developed, the following explanation has been found useful inunderstand ng and working with these constructions. Thermistor pair 11is taken as an example. Initially both the PTC thermistor P11 and theNTC thermistor N11 are relatively cool. The PTC thermistor P11 thus hasa relatively low resistance while the NTC thermistor N11 has arelatively high resistance. Accordingly, only a relatively small portionof the supply voltage appears across the PTC thermistor P11. Although arelatively large portion of the supply voltage thus appears across theNTC thermistor N11, only a relatively small current flows through theseries-connected pair due to the relatively high resistance of the NTCthermistor. In this state both of the thermistors are outside of theirnegative resistance regions and the pair is stable in this initialequilibrium condition.

If, however, heat is applied from an outside source to the NTCthermistor N11, as by means of the triggering thermistor PTI, thisheating has the effect of lowering the peak in the equilibrium NTCvoltage/current characteristic and NTC thermistor N11 thus passes intoits negative resistance region at the applied voltage. If NTC thermistorN11 is triggered in this way, it self-heats and tends to seek a newequilibrium at a higher temperature as determined by the electrical loadin series with it. However, as NTC thermistor N11 self-heats toward thenew equilibrium which it would acquire if its series load remainedconstant, the effect of this self-heating is to lower the resistance ofthe NTC thermistor N11 causing an increased portion of the sourcevoltage to be applied to the series-connected PTC thermistor P11. Thehigher voltage applied to the PTC thermistor P11 causes it to pass intoits negative resistance region and to begin a selfheating phase also.The load in series with the NTC thermistor N11 thus does not remainconstant.

The heat generated by the self-heating of PTC thermistor P11 is coupledto the NTC thermistor N12 in the adjacent pair of thermistors 12 so thatit also begins to self-heat. In addition to starting a bistableswitching cycle in the thermistor pair 12, the self-heat then generatedby NTC thermistor N12 is coupled back to PTC thermistor P11 and theresultant total heat increases the resistance of PTC thermistor P11 toan extent which sharply reduces the current flowing through theseries-connected pair 11.

This drop in current causes the NTC thermistor N11 to stop itsself-heating action and to revert to its original, relatively coolequilibrium condition in which it exhibits a high resistance. Theincreased resistance of the NTC thermistor N11 further reduces thecurrent through the series circuit 11 so that the PTC thermistor P11also then passes out of its negative resistance region and reverts toits original, relatively cool equilibrium state. With both thermistorsin their stable states, the circuit 11 is effectively reset and willremain quiescent until the NTC thermistor is again triggered either bythe application of external heat as described above or by some othermethod. In the meantime a cycle of bistable electrothermal switching hasbeen initiated in the thermistor pair 12. During the switching cycle ofthe thermistor pair 12, the selfheating phase of PTC thermistor P12 willinitiate a bistable cycle of operation in the thermistor pair '13 whichin turn initiates a similar cycle in pair 14, etc. Thus it can be seenthat a string of bistable switching operations is propagated down theplurality of thermistor pairs or stages, each successive cycle beingdelayed in time with respect to the preceding cycle. The time requiredfor completion of each bistable switching cycle is determined by thethermal inertia and dissipation characteristics of the thermistors whichmake up the pair. However, it may in general be noted that the periodsrequired are typically much longer than those encountered or practicallyobtainable in wholly electronic components of similar size. Further, thetime periods can vary over a wide range from stage to stage in the sameapparatus.

Since the triggering of each successive pair or stage by the precedingstage is provided by thermal coupling between the stages, it can be seenthat all of the stages can operate at essentially the same power levelor that a low power stage may even trip a somewhat higher powered stage.Further, the stages which are thermally coupled may be electricallyisolated from one another and powered from dilTerent sources.

The sequence of operations provided by the apparatus of FIG. 1 willrepeat continuously if the stages are coupled to form a loop in whicheach stage in the loop trig gers a following stage. In the apparatus ofFIG. 1, an

NTC thermistor N11A is thermally coupled to PTC thermistor P18 and canbe electrically connected in parallel with the NTC thermistor N11 bymeans of switch SW1. When NTC thermistor NllA is thus connected into thecircuit, it can take the place of thermistor N11 in bistable switchingwith the PTC thermistor P11. PTC thermistor P11 can thus function in itsswitching mode with either of the NT C thermistors N11 or N11A andtriggering of such switching operations can be initiated by externallyapplying heat to either thermistor N11 or thermistor N11A. Since thequiescent or stable state for thermistors N11 and N11A is the cool orhigh resistance state, that one of the two NTC thermistors which is notfunctioning in a switch ing 'cycle with PTC thermistor P11 does notappreciably load or affect the switching operation involving the other.A sequence of switching operations started by closing pushbutton PBIwill thus continue indefinitely when the switch SW1 is closed. When thesequence of operations reaches the thermistor pair 18, the heating ofPTC thermistor P18 will apply heat to the NTC thermistor N11A, therebytriggering another complete sequence of operations. This repetitivecycle of operations continues indefinitely until it is stopped byopening the switch SW1, thereby breaking the loop by severing theconnection between NTC thermistor N11A and PTC thermistor P11 whichprovides a predetermined point of interruption of the sequence.

As the various thermistor pairs 11-18 operate in timed succession, itcan be seen that the apparatus of FIG. 1 is suited for controllingoperations which are to be performed in a given sequence. The occurrenceof any or all of the successive steps within the overall sequence ofelectrothermal cycles may be sensed in various ways for control orsignalling purposes. As the current flowing through eachseries-connected pair of thermistors varies during its switching cycleof operation, the occurrence of such an operation may be detected bycurrent-sensing means connected in series with the thermistors. Aparticularly simple such current sensor is a low resistance incandescentlamp such as those indicated at 21 and 23 in the thermistor pairs 14 and18. Lamps 21 and 23, being of low resistance, do not appreciably affectthe switching mode of operation of the series pairs as describedpreviously.

As the voltage across either one of the constituent thermistors in eachseries-connected pair varies as the pair goes through its switchingcycle of operation, such a cycle of operation may also be detected bysensing the voltage across one of the thermistors. A particularly simpleform of voltage-sensing device is a high resistance incandescent bulbsuch as indicated at 25 connected across the NTC thermistor N16 inthermistor pair 16. Being of high resistance, lamp 25 does notappreciably load or interfere with the operation of the thermistor pair16 but will vary in brightness as the switching progresses. Lamps suchas 21, 23 and 25 may be used for visual indication of the sequencersprogress or, by driving suitable photosensitive devices, may be used forprogramming or con trolling outside operations which are to be performedin sequence.

The occurrence of an electrothermal switching cycle within any one ofthe stages may also be detected by sensing the heat dissipated from thethermistors in that stage. In FIG. 1, for example, a sensing thermistorTH1 is thermally coupled to the PTC thermistor P18 in the pair 18 forsensing the occurrence of an electrothermal switching cycle in thatpair. As is apparent to those skilled in the art, the changes inresistance of thermistor TH1 may be employed to effect variousprogramming operations. Since the sensing thermistor TH1 is notelectrically connected to the electrothermal timing circuitry but isonly thermally coupled thereto, it can be seen that electrical isolationcan be maintained between the controlling and controlled circuits.Thermochromic paint may be employed to obtain a temperature responsive,visual indication of the sequencers progress.

As there is a time delay between the operation of each successive stage,which delay depends upon the thermal characteristics of the constituentthermistors as noted previously, it can be seen that the apparatus ofFIG. 1 is suited for timing, sequencing and programming appli cations,particularly those involving long intervals.

In the apparatus of FIG. 1, the thermistors which are thermally coupledto each other do not share a common connection and thus the thermalcoupling must be arranged so as to maintain electrical isolation betweenthe thermally coupled thermistors. As noted previously, however, each ofthe thermistor pairs may be run on DC of either polarity or on AC. Theapparatus illustrated in FIG. 6 is similar to that illustrated in FIG. 1and comprises a plurality of pairs of thermistors 3148, each pairincluding a PTC thermistor, P31P38 respectively, and an NTC thermistor,N31-N38 respectively. In this construction, however, every other pair isreverse-connected with respect to the supply leads L1, L2. Thus, thethermistors of opposite coefficient type which are thermally coupledtogether to provide the transfer of triggering energy now have a commonelectrical connection. For example, PTC thermistor P31 and NTCthermistor N32 which are thermally coupled together are also bothconnected to the supply lead L2. With this common electrical connection,the thermal coupling between the thermistors need not maintainelectrical isolation and can be relatively easily provided by bringingthe two thermistors into contact at their common electrical connectionas in the construction illustrated in FIGS. 7 and 8.

Referring to FIGS. 7 and 8, the thermistors are assembled as a unit on aheader 41 of the type conventionally used for mounting transistors.Three leads 43, 45 and 47 extend through insulating hermetic seals inthe header. A small block 49 of PTC thermistor material is soldered to asomewhat larger block 51 of NTC material as indicated at 53 and lead 43is also soldered to this interface. Lead 45 is soldered to the outsidesurface of the block 51 while the lead 47 is soldered to the outsidesurface of the PTC block 49. As may be seen, this construction providesall of the required electrical connections while affording good thermalcoupling between the two blocks of thermistor material. It should benoted that the two thermistors which are thus assembled as a unit do notconstitute a single electrothermal switching stage or pair, but ratherthe two thermistors are component parts of two adjacent pairs in thesequencer of FIG. 6.

As noted previously, the behavior of an NTC thermistor with respect tochanges in current is in many ways analogous to the behavior of a PTCthermistor wtih respect to changes in voltage. Accordingly, the roles ofthese thermistors in timing apparatus according to the present inventionmay be, in a sense, interchanged if parallel and series connections aresimilarly interchanged and if the resultant pairs are operated onsubstantially constant current rather than substantially constantvoltage. Similar changes must also be made in the sequence initiatingand feedback circuits.

A sequencer constructed according to this analogy is illustrated in FIG.9. This apparatus comprises four pairs of thermistors 61-64. Each suchpair or stage includes a PTC thermistor, P61-P64 respectively,electrically connected in parallel with an NTC thermistor, N61-N64respectively. The parallel connection of PTC thermistor P61 with NTCthermistor N61 is completed through one side of a S.P.D.T. switch SW3for a purpose described hereinafter. The NTC thermistors N61-N63 in thepairs 61-63 are thermally coupled to the PTC thermistors P62P64respectively, as indicated by the broken lines linking thermally coupledthermistors. The PTC thermistor P61 in the first pair 61 is thermallycoupled to an NTC triggering thermistor NT which is connected inparallel with a voltage limiting resistor R3 and a normally closedpushbutton switch PB3 to constitute a triggering circuit 65. The NTCthermistor N64 in the last pair 64 is thermally cou- 7 pled to a PTCfeedback thermistor P61A. Feedback thermistor P61A may be selectivelyplaced in series with the PTC thermistor P61 in the first pair 61 byreversing switch SW3.

The thermistor pairs 61-64 and the triggering circuit 65 are connectedin series across a pair of supply leads L and L6 for providingsubstantially constant current to the pairs. Leads L5 and L6 may beconnected to a conventional constant current source although, asunderstood by those skilled in the art, a series connection of a largenumber of units such as the pairs 61-64 will inherently cause each unitto be supplied with essentially constant current despite nominalvariations in the resistance of that one unit. The parallel-connectedthermistors of opposite temperature coefficient types are essentiallycurrent-coupled in that, if the current passing through one of themchanges for any reason, the current through the other must change incomplementary fashion in order to maintain the total current at thepredetermined constant level.

According to a presently accepted theory, the electrothermal switchingoperation of each one of the pairs 61- 64 proceeds substantially asfollows. The pair 61 is taken as an example and it is assumed that theswitch SW3 is in the position shown. Assuming that the level of currentis appropriately adjusted, the pair will normally remain in a quiescentstate with both thermistors being relatively cool. In this state the PTCthermistor P61 has a relatively low resistance so that it conducts mostof the substantially constant current provided from the source.

If the PTC thermistor P61 is heated by a source external to the pair,e.g., by operating switch PB3 to cause triggering thermistor NT to heat,this heating has the effect of lowering the peak of the curve shown inFIG. 4 so that the current being carried by PTC thermistor P61 issuflicient to drive it into the negative resistance region and to causeit to regeneratively self-heat. The PTC thermistor P61 self-heats towardits transition temperature where its resistance increases sharply. Asthe resistance of the PTC thermistor increases, the voltage across theparallel-connected thermistor pair increases so that the voltage acrossthe NTC thermistor surpasses the peak in its voltage/ currentcharacteristic, thereby causing it to also self-heat regeneratively.Heat generated in the NTC therm istor N61 is coupled to the PTCthermistor P62 in the next stage or pair and initiates an electrothermalswitching cycle in that pair starting with a self-heating phase of PTCthermistor P62. The heat generated by PTC thermistor P62 is coupled backto the NTC thermistor N61 which triggered it and the total heat appliedto and generated within NT C thermistor N61 causes its resistance todrop to a. low level. The drop in resistance in thermistor N61 causes itto shunt current away from the PTC thermistor, thereby stopping theregenerative self-heating of the PTC and allowing it to revert to itsoriginal stable state. As the PTC thermistor reverts, its droppingresistance reduces the voltage across the NTC thermistor, so that theNTC thermistor also can no longer regeneratively selfheat and thus alsoreverts to its original stable state. This quiescent condition of thepair will then last until the pair is again triggered by applyingexternal heat to the PTC thermistor.

The thermal coupling between the NTC thermistors in pairs 61-63 and thePTC thermistors in pairs 62-64 causes the occurrence of self-heatingphase in one of those NTC thermistors to trigger or initiate anelectrothermal switching cycle in the next stage or pair. Thus, once asequence of electrothermal operations is triggered by the heating ofthermitsor NT, it propagates through the entire series of thermistorpairs.

The sequence of operations of the apparatus shown in FIG. 9 can becaused to repeat indefinitely by reversing the position of switch SW3thereby placing the feedback thermistor P61A in series with the PTCthermistor P61 of the first pair 61. As both of the thermistors P61 andP61A have a low resistance when cool, the main portion of the constantsupply current must still flow through this arm of the parallel circuitwhen the stage is in its quiescent stage, and thus thermistor N61 willnot self-heat unless this stage is triggered. However, an increase inresistance in either of thermistors P61 or P61A, such as may be producedby external heating, will initiate an electrothermal swtiching cycle ofoperation as previously described. The untriggered one of the two PTCthermistors P61 or P61A will not significantly interfere with theswitching operation involving the other thermistor since the untriggeredthermistor at all times remains at a low impedance level. Accordingly itcan be seen that, when the last of the pairs (64) passes through itselectrothermal cycle, the self-heating phase of NTC thermistor N64 willtrigger an electrothermal cycle in the first stage 61 by applying heatto the PTC feedback thermistor P61A.

' Thus the circuit of FIG. 9 will function in the manner of a ringcounter or ring pulser. For control, programming or indication purposes,the stage which is operating at any given moment can be detected orsensed in the various manners described previously with reference toFIG. 1. That is, the current though a thermistor may be sensed, thevoltage across thermistor pairs may be sensed, or the heat dissipatedfrom one of the thermistors may be sensed to efiect electricallyisolated signal coupling.

While the operations of the various embodiments have been explained withreference to a theory of operation which assumes a bistable mode ofoperation, it should be understood that pairs of thermistors accordingto this invention may also be constructed to behave according to aso-called monostable or one-shot mode of operation in which the stage ofitself reverts to its original condition a predetermined interval afterbeing triggered.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. Electrothermal timing apparatus comprising:

a plurality of pairs of pairs of thermistors, one of the thermistors ineach of said pairs being of a first temperature coefiicient type and theother of said thermistors being of opposite temperature coefficienttype, at least one of said thermistors having a negative resistanceregion of operation;

circuit means interconnecting the thermistors in each pair with eachother to provide electrical coupling therebetween and forinterconnecting each of said pairs of thermistors with an electric powersource whereby heating of the thermistor of said first coefiicient typein one of said pairs will initiate an electro thermal cycle in whichsaid one and then said other of said thermistors in said one pair passthrough self heating phases in sequence;

means for thermall coupling the thermistor of said opposite coeflicienttype in a first one of said pairs to the thermistor of said firstcoefiicient type in a second one of said pairs whereby the occurrence ofan electrothermal cycle in said first pair of thermistors will initiatean electro-thermal cycle in said second pair of thermistors.

2. Timing apparatus as set forth in claim 1 wherein the thermistors ineach pair are connected in series thereby providing voltage couplingtherebetween.

3. Timing apparatus as set forth in claim 2 wherein said pairs ofthermistors are connected in parallel across an electric power sourceproviding a substantially constant voltage to each of said pairs.

3,457,429 9 1G 4. Timing apparatus as set forth in claim 1 wherein the9. Electrothermal timing apparatus comprising: thermistors in each pairare connected in parallel thereby a plurality of pairs of thermistors,each pair including providing current coupling therebetween. a PTCthermistor in series with an NTC thermistor,

5. Timing apparatus as set forth in claim 4 wherein at least said PTCthermistor having a negative resaid pairs of thermistors are connectedin series to an 5 sistance region of operation; electric power sourceproviding a substantially constant circuit means connecting each of saidpairs across a current to each of said pairs. source providing asubstantially constant voltage 6. An electrothermal sequencercomprising: whereby heating of the NTC thermistor in one of a pluralityof pairs of thermistors, one of the thermissaid pairs will initiate anelectro-thermal cycle in tors in each of said pairs being of a firsttemperature coefficient type and the other of said thermistors being ofopposite temperature coefiicient type, at least one of said thermistorshaving a negative resistance region of operation;

circuit means interconnecting the thermistors in each pair with eachother to provide electrical coupling therebetween and forinterconnecting each of said pairs of thermistors with an electric powersource whereby heating of the thermistor of said first cowhich the NTCthermistor and then the PTC thermistor in that pair pass throughself-heating phases in sequence;

means for thermally coupling the PTC thermistor in efficient type in oneof said pairs will initiate an electrothermal cycle in which said oneand then said other of said thermistors in said one pair pass throughself-heating phases in sequence;

means for thermally coupling the thermistor of said opposite coeflicienttype in all but one of said pairs 10. Timing apparatus as set forth inclaim 9 wherein said PTC thermistor has a resistance characteristichaving a transition temperature above which the resistance of said PTCthermistor rises relatively abruptly.

11. Electrothermal timing apparatus comprising:

a plurality of pairs of thermistors, each pair including to thethermistor of said first coefiicient type in respective ones of theother pairs thereby defining a series of said pairs whereby theinitiation of an electrothermal cycle in the first one of said pairs ofthermistors in said series will initiate a sequence of electro-thermalcycles in the other pairs of thermistors in said series.

An electrothermal ring counter comprising:

a plurality of pairs of thermistors, one of the thermistors in each ofsaid pairs being of a first temperature coefficient type and the otherof said thermistors being of opposite temperature coefficient type, atleast one of said thermistors having a negative resistance region ofoperation;

circuit means interconnecting the thermistors in each pair with eachother to provide electrical coupling therebetween and forinterconnecting each of said a PTC thermistor in parallel with an NTCthermistor, at least said PTC thermistor having a negative resistanceregion of operation;

circuit means connecting said pairs in series across a source thereby toprovide a substantially constant current through each pair wherebyheating of the PTC thermistor in one of said pairs will initiate anelectrothermal cycle in which the PTC thermistor and then the NTCthermistor in that pair pass through selfheating phases in sequence;

means for thermally coupling the NTC thermistor in each of a pluralityof said pairs to the PTC thermistor in a respective other one of saidpairs thereby defining a series of said pairs whereby the initiation ofan electrothermal cycle in the first one of said pairs in said serieswill initiate a sequence of electrothermal cycles in the other pairs insaid series.

12. Timing apparatus as set forth in claim 11 wherein said PTCthermistor has a resistance characteristic having a transitiontemperature above which the resistance of said PTC thermistor risesrelatively abruptly.

pairs of thermistors with an electric power source whereby heating ofthe thermistor of said first coefiicient type in one of said pairs willinitiate an electro-thermal cycle in which said one and then said otherof said thermistors in said one pair pass through self-heating phases insequence;

means for thermally coupling the thermistor of said opposite coefiicienttype in each of said pairs to the References Cited UNITED STATES PATENTS3,175,090 3/1965 Reis et al.

thermlstor of said first coefficient type in a respective 3 259 7947/1966 Krawitz X one of the other pairs whereby the initiation of an3343004 6 electro-thermal cycle in one pair of thermistors will 9/19 7oushmsky n 338 25 X initiate a repeating sequence of electro-thermalcycles in said pairs of thermistors. 8. Apparatus as set forth in claim7 including switch means for selectively breaking the interconnectionbetween the thermistors in one of said pairs of thermistors to provide apredetermined point of interruption of said sequence.

ROBERT K. SCHAEFER, Primary Examiner T. B. JOIKE, Assistant Examiner US.Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. Dated22 Inventor(s) Francis uiting It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the heading to the printed specification line .1

for "(3 4 Forest St., Attleboro, Mass. 02703)" read --assignor to TexasInstruments Incorporated, Dallas, Tex., acorporation of Delaware--SIGNED ANU SEALED DEC 2 31969 (SEAL) Attest:

WILLIAM E. sdmmm, .m. Amsting Officer oomissioner of Patents

